In a power station including such a boiler, it is necessary to exercise accurate monitoring on the three-dimensional distribution of power within the core of the reactor. Such monitoring makes it possible both under normal operation and during accidental transients to make sure that safety criteria for the boiler are satisfied.
Thus, an authorized operating range for the core is defined by a network of straight line limits plotted in a plane (see FIG. 1) in which the coordinates are nuclear power and axial power difference. The axial power difference parameter, referred to below by the letters DI, is defined as being the difference between the power PH measured at the top of the core and the power PB measured at the bottom of the core. EQU DI=PH-PB
This parameter is representative of the axial distribution of power within the core.
By keeping the operating point of the core within the domain delimited in this manner, it is possible to ensure, inter alia that safety criteria are satisfied in the event of accidental loss of primary cooling fluid.
In addition, in order to avoid excessive variations in the temperature of the primary heat exchange fluid which cools the core, a program is defined for a reference temperature as a function of power level. When there is a difference between the measured temperature and the reference temperature, a regulation system is capable of acting automatically on the core control clusters in such a manner as to correct the difference. When this temperature regulation system is in action, the clusters are said to be in automatic mode. A cooling transient causes clusters to be extracted, and a heating transient causes them to be inserted.
A problem arises with respect to this regulation:
During certain accidental transients that cause the primary fluid to cool down suddenly, the temperature regulation system will cause the control clusters to be raised quickly if they are in automatic mode. However, such sudden extraction of the absorbent clusters has two consequences which are shown in one particular case by two curve segments C1 (solid line) and C2 (dashed line) in FIG. 1:
firstly, core reactivity increases, thereby raising the nuclear power DT as plotted up the Y axis of FIG. 1; and
secondly, power distribution rises towards the top of the core, thereby increasing the axial power difference DI which is plotted along the X axis. In FIG. 1, the authorized operating domain is represented by an outer limit FA which is constituted by straight line segments.
In the event of a pre-accidental situation of the core represented by a point situated to the right of this operating domain, the nuclear power and axial power difference excursion may give rise to the operating point moving a considerable distance outside the domain. The characteristic evolution of such a cooling transient is shown by segments C1 and C2. A detailed analysis of this type of transient has shown that the core safety limits are approached and even exceeded without intervention of one of the reactor protection systems necessarily being guaranteed.
A particular object of the present invention is to limit the risk that may result from an uncontrolled excursion of the reactor operating point outside said operating domain in the event that said excursion is related to a rapid extraction of the temperature regulating group of clusters.
The invention is based on creating an inhibit signal for inhibiting instructions to extract the temperature regulation group whenever the reactor operating point leaves the operating domain dangerously. With conventional reactors, leaving the operating domain dangerously is equivalent to leaving it to the right in FIG. 1.